The present invention relates to signaling circuits adapted to be used in connection with railroad tracks and, more particularly, to a carrier frequency, shunt type of track circuit.
Carrier frequency signaling systems have been employed for many years on railroad lines and such systems provide advantageous operation inasmuch as a large number of signaling channels can be accommodated over the same transmission path. Such systems involve track circuits which are designed so that they rely on a shunting effect between the rails through the body or frame members of a railroad train. Thus, when there is no train present in a particular block, there is no shunting effect present and normal transmission takes place over the circuit between a transmitter and a suitable receiver or receivers. However, as soon as the train enters the particular block the aforesaid shunting takes place with the result that the receiver no longer provides its normal output signal, thereby actuating an appropriate warning device or providing over "safe" action.
Although the aforesaid systems have been known to function well under conditions of moderate or heavy traffic on a railroad line, it has been observed that track circuits on light traffic lines do not always shunt reliably because an insulating film tends to develop on the rails during the long time gaps encountered between the passage of successive trains. It has further been observed that when a moving train fails to cause the track relay to release continuously, that same train when stopped does shunt the track circuit completely. From these observations it may be inferred that a moving train shunts just as well as one that is stopped, but not one hundred per cent of the time.
The foregoing hypothesis has been tested on several occasions and the problem or difficulty encountered has been overcome by means of a principal feature of the present invention.
Accordingly, it is a primary object of the present invention to ensure great reliability in the operation of a carrier frequency, shunt type signaling system on a lightly traveled railroad line.
Another object is to overcome the difficulty that occurs on a lightly traveled line when, because of the presence of an insulating film, a moving train tends not to produce a sufficiently long shunting effect on a continuous basis but rather only on an intermittant basis.
Another object is to accomplish the foregoing in a failsafe manner.
In fulfillment of the above stated objects, the present invention provides the feature of a quick-acting shunt detector which is incorporated within a more or less standard track circuit receiver. This quick-acting shunt detector functions so as to interrupt the receiver output long enough to release the vital track relay and to produce the appropriate warning effects even though the shunt is of extremely short duration, i.e., of the order of 15 milliseconds. Without the presence of the quick acting shunt detector, the conventional receiver might ignore the occurrence of one or more short duration shunts as these occur. The reason for this is that it takes 300 milliseconds of shunting in the normal case to cause the vital relay to release.
Effectively then, the shunt detector of the present invention acts by reason of its internal logic to detect a shunt as short as 15 milliseconds and enhance or stretch it to a predetermined time period. This predetermined time period has been selected from several considerations to be of the order of 2.5 seconds. Accordingly, any time there appears short duration shunting of the track system within a 2.5 second period, the vital relay is kept in the released state, that is, the relay remains down so as to keep an emergency device activated.